I'll assume that you're using a standard Access form.
You can get up and running (sort of) programmatically by using something along the lines of:
If Not Me![TextX] Like "*@*" Or Not Me![TextX] Like "*.*" Then
MsgBox ("Invalid e-mail address")
Change the two [TextX] entries in the above code to the name of your text field. This validates the @ sign and makes certain you have a . in the address.
Note that there are potentially other issues involved, depending very much on how you want your forms to run and if there are other fields you want to automatically leap to after this one has been filled.